It is well known to provide in-line galvanising of steel sections such as round tube where the section is formed to substantially the desired shape and then passed through an in-line galvanising bath or trough to provide the coating of galvanising material which is typically zinc. This works well for most closed sections but problems are encountered with some open profile shapes, such as channels, in obtaining an even coating of galvanising material after the bath due to the difficulty in quickly blowing excess material from various areas of the section.
It has also been known to form open structural profiles of galvanised steel by taking pre-galvanised flat steel strip and roll forming that strip to the desired profile, e.g. angle or channel sections by a conventional roll forming process. The bending of the flat strip in the roll forming process does however impose significant elongations of the outer surface of each corner formed when the strip is thick enough to form a structural profile. (Throughout this specification the term "structural profile" is taken to refer to profiles formed from strip having a thickness generally greater than 2 mm.) To deal with this situation, the industry standard has been to use a zinc coating where the galvanising material incorporates alloys such as aluminium and where the base metal strip to which the galvanised coating has been applied is formed either by the cold rolling process or by hot rolled, pickled and oiled. The combination of the cold rolled steel base layer and significant percentages of alloy in the zinc coating when correctly applied according to known processes results in a thin and ductile galvanising layer on the base metal which can withstand the significant elongation required in the bending of corners during the roll forming of the structural profile.
However there are instances in the manufacturing process where the zinc coating may not always be correctly applied according to the most desirable parameters and in such cases less ductile coatings than desired result which in some instances can crack through significant elongation of the outer surface during the roll forming process. This problem may also exist where the strip has been galvanised in simpler galvanising operations which do not use aluminium alloys or where the parent material has a surface which can be regarded as highly reactive to the galvanising process (such as shot blasted strip).
When using both cold rolled and hot rolled base material it is desirable to provide a method of forming a structural section where some allowance can be made for less than ideal zinc coating parameters so that forming of the material to the desired shape after galvanising has taken place does not result in unacceptable levels of cracking in the zinc layer.